Seriously, this is the direction I'm thinking of going. It seems like a much cheaper option than buying another Garmin, plus it will allow me to play around with Android while my iPhone is still under contract.

Someone, please lead us

Android offers a lot of choice, and that's your first hurdle. you need to decide what you want and then figure out how to get there. Me... First, I don't care about routing so tile-based local maps are the best option for me. Second, I'm experienced in manipulating my own mapping data... so I want the option of custom maps and that raises the technical bar considerably. Most don't need this. Others will work differently. What I'm trying to get at is that there isn't going to be a single cookie-cutter answer to what you need. Sorry.

However, to answer your question, no. You can't just point the old phone at google maps and get what you want, not without the phone being on a data plan and you being in a cell service area. You need to load the maps on the device and if you want routing then you need an app on your device that offers routing. Yes, they exist. Also, you will need to figure out mounting and power for the device. And, finally, you will soon realize that all screens are not equal. Daylight readability will depend on your device and the maps you run.